Substantially water-insoluble compounds useful in imaging are generally incorporated into imaging materials as dispersions or emulsions. Such compounds include inter alia filter dyes used to prevent transmission of unwanted light from one layer in a photographic element to another and thermal transfer dyes used to create dye images in a thermal dye transfer process.
In many cases, the water-insoluble compound useful in imaging is dissolved in one or more organic solvents, and the resulting oily liquid is then dispersed into an aqueous solution containing, optionally, emulsifiers or dispersing aids such as surfactants and/or hydrophilic colloids such as gelatin. Dispersal of the oily liquid into the aqueous medium is accomplished using high shearing rates or high turbulence in devices such as colloid mills, homogenizers, or ultrasonicators.
In the art of dispersion making, the use of organic solvents has traditionally been considered necessary to achieve small particle sizes, to achieve stable dispersions, and to achieve the desired reactivity or function of the compound useful in imaging. Many compounds useful in imaging cannot be dispersed in the above manner, however, because of their poor solubility in most organic solvents. Others may have sufficient solubility in organic solvents, but it may be desirable to eliminate the use of the organic solvent to, for example, reduce coated layer thickness, to reduce undesirable interactions of the organic solvent with other materials in the imaging element, to reduce risk of fire or operator exposure in manufacturing, to obtain the desired light absorption characteristics, or to improve the sharpness of the resulting image, which is adversely affected by the presence of the organic solvent.
In view of these disadvantages, it has been proposed in the art to use solid particle dispersions in photography, for example, solid particle filter dye dispersions, in U.S. Pat. No. 4,294,916 to Postle et al, solid particle filter dye dispersions U.S. Pat. No. 4,294,917 to Postle et al, and U.S. Pat. No. 4,940,654 to Diehl and Factor. Techniques for making solid particle dispersions are very different from the techniques used to make dispersions of oily liquids as described above. Typically, solid particle dispersions are made by mixing the crystalline solid of interest with an aqueous solution that may contain one or more grinding aids or stabilizers. Particle size reduction is accomplished by subjecting the solid crystals in the slurry to repeated collisions with hard, inorganic milling media, such as sand, spheres of silica, stainless steel, silicon carbide, glass, zirconium, zirconium oxide, yttria-stabilized zirconium oxide, alumina, titanium etc., all of which fracture the crystals. The bead sizes typically range from 0.25 to 3.0 millimeters (mm) in diameter. Ball mills, media mills, attritor mills, jet mills, vibratory mills, etc. are frequently used to accomplish particle size reduction.
Unfortunately, both the size reduction of crystalline compounds useful in imaging and the stabilization of the resulting dispersions are much more difficult than the emulsification of liquids or the stabilization of conventional liquid droplet dispersions. Traditional dispersants or stabilizers such as anionic or nonionic alkyl or aryl surfactants, as disclosed in the above-noted U.S. Pat. Nos. 4,294,916, 4,294,917 and 4,940,654, tend to adsorb much more readily to liquid surfaces than to the surfaces of solid particle surfaces and the use of such traditional dispersants or stabilizers as grinding aids in the making of solid particle dispersions frequently results in extraordinarily long milling times to achieve a given particle size reduction. Furthermore, traditional stabilizers tend to promote unwanted crystal and/or needle growth. Such particle growth is undesirable since it reduces the covering power of the compound useful in imaging in the coated layers of an imaging element, while the presence of needle-like crystals results in filter plugging and poor manufacturability. Filter dyes are particularly susceptible to needle growth when mixed, prior to coating, with conventional dispersions containing organic solvents.
Solid particle dispersions of photographically useful materials have been disclosed in UK Patent No. 1,570,362 to Langen et. al. and U.S. Pat. No. 4,006,025 to Swank. In U.K. Patent No. 1,570,362, monomeric, oligomeric, and polymeric alkylaryl sulfonates are disclosed as dispersing agents for solid spectral sensitizing dye and coupler dispersions. In U.S. Pat. No. 4,006,025, very low levels of poly(vinyl pyrrolidone) are disclosed to be useful as grinding aids in the production of solid particle dispersions of anionic, cationic, or zwitterionic cyanine spectral sensitizing dyes, and it is further disclosed that poly(vinyl pyrrolidone) can be added to the resulting dispersions after milling as an antiflocculant. We have found that the procedures described in Langen and Swank result in dispersions having unacceptably large particle size or are unstable, especially to undesired particle growth.